I'd like to be able to update my watches with the NIST atomic time broadcast, which unfortunately we don't have here in Australia. I've looked at the format of the broadcast and think it should all be quite achievable except the actual RF, which I have no experience in. I would like to know if anyone experienced with transmitter design is out there and can confirm for me if this simple idea might work.
To my way of thinking 60 Khz is extremely low frequency, so I was thinking of just using a 555 timer (wired for 50% duty cycle), with the output hooked through a capacitor (to make it AC) and a coil (to make it more sine-waveish) to a short length of wire for an antenna. Would that constitute a transmitter?
Make the coil big, leave off the short length of wire, and put your stuff inside the coil to adjust the time.
Then turn the power way down to avoid interfering with whatever Oz uses
60kHz for.
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It might work however the coverage area will be very small.
Generate the signal by a microcontroller and use a push - pull pair of MOSFETs as the power stage. The biggest problem with the VLF is the transmitting antenna. The efficient antenna has to be several kilometers in size. Alternatively, you can use the frame antenna, however even if you pump hundreds of watts on it the range will be only about 20...30 meters or so.
Vladimir Vassilevsky DSP and Mixed Signal Consultant
On Feb 2, 3:27=A0pm, snipped-for-privacy@powerup.com.au wrote: > I'd like to be able to update my watches with the NIST atomic time > broadcast, which unfortunately we don't have here in Australia. I've > looked at the format of the broadcast and think it should all be quite > achievable except the actual RF, which I have no experience in. I > would like to know if anyone experienced with transmitter design is > out there and can confirm for me if this simple idea might work. >
I have wind up pendulum clocks that are more accurate than your 'reference'. You do know the carrier frequency is _extremely_ accurate.
Ok, think about it... How accurate does the RECEIVER oscillator have to be to make NIST work? IF the time code is encoded accurately, you won't be off by more than the oscillator peroid, which is plenty good for setting a watch.
But I agree that you should not transmit on this frequency. I don't want anyone setting MY clock.
Which is ideal, since it would constitute an unlicensed transmitter, and the Australian authorities are probably no more fond of that than anybody else's government. In point of fact, I'd suggest setting up an old microwave oven carcass (or other metal box) with the transmitter and watches placed inside, and only powering it up for a brief period to set the watches. Hardly needs efficiency or power.
Thanks all for the input. My understanding is what I am doing is legal in Australia, as long as I don't exceed the maximum EIRP for LIPDs in this frequency range. With my current plans I don't think that is possible. I want to use simple components I have available to me and I am familiar with. The frequency must be 60Khz, and I would worry placing the watch into a big inductive loop might damage the watch/ receiver.
I think I'll put the oscillator together on a proto board and see what comes of it. Another thought, is more voltage better for transmission? I could hook up the 555's output to the secondary winding of a transformer and hook one side of the primary to ground and the other to my antenna. Maybe a mains transformer would be no good, but one from a switch-mode power supply might be? Or maybe an audio transformer?
On a sunny day (Sun, 3 Feb 2008 12:56:14 -0800 (PST)) it happened snipped-for-privacy@powerup.com.au wrote in :
This is how to drive a current loop at 60kHz:
60kHz generator - amplifier --low volt lightbulb - capacitor -- L- | ) 1 or more turns around the room | /// The lightbulb forms a constant current source, you need to figure C and L so the L + Lloop, together with C, is in resonance. Use a high voltage C, there will be huge voltages. A good audio amp IC will do for the amplifier.
I have used this (with license) for translation systems in conference halls. I think you will need a license in most places. Something like 10 W should be enough.
The COTS are designed for 60KHz. You can't change the frequency.
I wonder if the original poster tried to listen to 60KHz on a radio. The signal might be there, but too weak to drive something COTS. If that is the case, maybe a repeater of sorts would work. The signal itself is very low bandwidth. In the states, it is no problem to pick it up on a radio, especially with a CW filter and loop antenna.
Most of these atomic clocks have puny antennas. I have a Sony alarm clock that has the NIST built in. The antenna is nearly the size of a computer mouse. How the watches work is beyond me.
In message , snipped-for-privacy@powerup.com.au writes
A loop is probably the easiest way of coupling the watch to the transmitter without radiating nuisance signals to all and sundry.
Your biggest problem will be hitting the required 60kHz carrier frequency accurately enough for the high Q clock receiver to even notice. .
You do realise that you have to modulate the carrier with the appropriate time code data for this to work. And if you have to do that then you may as well use a PIC with a 6MHz or 12Mhz quartz xtal (about $1) and divide that down . At least then you can get the carrier wave exactly on frequency spec and concentrate on modulating it.
Cheap hobbyist PIC kits might even come with all the bits for $10-20.
A 555 will drift like hell compared with what the receiver needs.
I doubt it is legal to jam 60kHz signals even in Australia, but so long as you keep the power low there is enough space that no-one will notice. Place the bits you want to tweak close to your transmitter and use the lowest power needed to do what you want.
You're overthinking this. If I understand the project, it's to set a watch...not recreate WWVB.
Some of us think it might be fun to do the experiment with someone else actually building it.
And probably more accurate than the spray can of cheese whiz in my fridge...and about as relevant.
The 555 is the carrier. All it has to do is stay in the bandpass of the receiver. So, the real question is, "what are the receiver requirements for carrier accuracy and stability?" I'm betting that the $19 WWVB atomic clock hanging on my wall has a pretty sloppy receiver.
While I'm at it, let's ask one more relevant question... WHEN does the watch update itself. I'm just guessing that the power consumption of the WWVB receiver might be orders of magnitude greater than the power consumption of the part that goes tick-tock. That suggests that the receiver might be OFF most of the time. Won't help to have a synchronization setup that's not near when the watch is listening. And what if it can't find a signal? Does it retry? How often? Does it ever give up and quit trying altogether? Battery drain may go through the roof. Or it may just stop trying to synchronize. Knowledge of the update algorithm might be critical here.
Most of us DO have access to an atomic clock. It's called GPS. For the rest of you, the time clock in your computer, synchronized to a time server, is plenty accurate to set a watch...my watch anyway...
The whole project is silly. If you've got a wrist watch that cost over $9.99 and needs to be set more than once a year, it's broke. Take it back. If you need action on a schedule that accurate, you need something more reliable than a human with an accurate watch to orchestrate it.
But who among us has never delighted in a project that others thought a waste of time?
The MSF (UK) radio-controlled clock that I disassembled contained a crystal used as the filter in the receiver, so it probably has a receiver bandwidth of only a couple of Hz. You wil need a crystal- controlled transmitter. Probably the easiest way is to use a microcontroller with an appropriate crystal (e.g. 6MHz ? 24MHz etc.) and then configure one of the counters inside the microcontroller to act as a divider. The micro could also generate the time code modulation. A coil of wire would be the most effective antenna, ideally with a capacitor across it so that it resonates at the right frequency.
The clock that I disassembled also contained a 32768Hz watch crystal which was used for the time-keeping most of the time. At the end of each hour it would turn on the radio receiver and wait until it received a time code with correct parity and then it would set the time and turn off the receiver. Unfortunately, when receiving pure interference it would have a 50% chance of receiving correct parity, at which point it would set the time to some nonsensical time like
34:81, because they did not think to check whether the hours were less than 24 or the minutes less than 60.
Whether or not building the proposed trnasmitter would be legal where you are, it is unlikely to be noticed amongst all of the BPL hash, and so as long as you don't go telling people about it, probably noone will care. If you were to build the whole arrangement inside a metal box with your watch, then that would quite likely be legal, and so for the purpose of this discussion that is what I will assume you did.
I met an engineer who (whilst a student) built a time code transmitter so configured that it would make all of the clocks at his university run roughly backwards, to the amusement of all.
See chrisgj198's post. If the bit pattern does not make some sense and have good parity, then the clock will not set.
The same data is also encoded in the WWV and WWVH transmissions at 5 MHz, 10 MHz, and 20 MHz. I consider it more reasonable for a watch to use one of those signals.
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